1. Field of the Invention
The present invention relates to a DFB laser driving device, a DFB laser driving method and a storage medium for use in a DWDM (Dense Wavelength Division Multiplexing) communication system and a device for testing the same or the like.
2. Description of the Related Art
In recent years, a dense wavelength division multiplexing (DWDM) communication system has been used in long-haul large-capacity communications.
In the DWDM communication system, the communications are achieved such that the wavelengths of a plurality of optical signals different in wavelength are multiplied in an optical fiber serving as a transmission path by using, as a carrier signal, a light beam emitted from a light source such as a distributed feedback laser diode (hereinafter referred to as a DFB laser) for emitting a laser beam in a single wavelength.
In the DWDM communication system, since the wavelengths of the optical signal and the carrier signal are synthesized at an input terminal of the optical fiber while the wavelength is divided at an output terminal of the optical fiber, it is necessary to strictly control the wavelength of the DFB laser as the carrier signal.
Furthermore, a plurality of optical amplifiers are provided on the transmission path, for amplifying the carrier signal, and therefore, it is necessary to strictly control also the optical power level of the carrier signal.
Referring to FIG. 4, explanation will be made on a conventional DFB laser driving device 200.
The DFB laser driving device 200 illustrated in FIG. 4 comprises a DFB laser 1, a Peltier device 2, a thermistor 3, a photodiode 4, a laser driving circuit 5, an optical output controlling D/A converter 6, a temperature control circuit 7, a CPU 90, a storage device 10, a storage medium 11 and an interface 12.
The DFB laser 1 is driven such that when optical output setting information is input into the CPU 90 via the interface 12, the CPU outputs an optical output control signal (a digital signal) to the optical output controlling D/A converter 6 based on the optical output setting information and data of an optical output level stored in the storage medium 12 inside the storage device 10.
Subsequently, the optical output controlling D/A converter 6 converts the optical output control signal (the digital signal) into an analog signal to output the analog signal to the laser driving circuit 5. The laser driving circuit 5 outputs a drive current to the DFB laser 1, and thereafter, the DFB laser 1 outputs optical output.
Furthermore, the temperature control circuit 7 cools the DFB laser 1 by applying a current to the Peltier device 2 based on a temperature controlling reference voltage input into an input terminal thereof and a feedback signal input from the thermistor 3, thereby controlling the DFB laser 1 at a constant temperature.
FIG. 5 graphically illustrates the relationships among the wavelength, optical output level and temperature of a laser beam radiated from the DFB laser 1.
As illustrated in FIGS. 5A and 5B, when the temperature of the DFB laser 1 is constant, the optical output level is attenuated according to a decrease in drive current of the DFB laser 1, and whereby the wavelength of the laser beam radiated from the DFB laser 1 is shifted toward a short wavelength side. In the meantime, as illustrated in FIGS. 5C and 5D, when the optical output level of the DFB laser 1 is constant, the temperature of the DFB laser 1 is decreased, and whereby the wavelength of the laser beam radiated from the DFB laser 1 is shifted toward a short wavelength side.
The relationship between the temperature and the optical output level of the DFB laser 1 generally shows that higher output is obtained at a lower temperature side. However, no predetermined regularity may be established between the temperature and the optical output level of the DFB laser 1 owing to variations in coupling efficiency between the radiated laser beam and the optical fiber (the transmission path) or the like caused by a change in wavelength of the radiated laser beam and a change in temperature of the DFB laser 1, as illustrated in FIG. 5E.
The DFB laser driving device 200 disposed at the input terminal of the transmission path needs to perform a “pre-emphasis” for regulating the optical output level of the carrier signal in order to keep a constant receiving level of the carrier signal assigned to the optical signal of each of the wavelengths at the output terminal of the transmission path in the DWDM communication system.
However, since the temperature of the DFB laser 1 is controlled at a constant value by the temperature control circuit 7 in the conventional DFB laser driving device 200, when the optical output level is regulated, there has arisen a problem of a change in wavelength of the radiated laser beam, as illustrated in FIG. 6.
Consequently, if the optical output level of the DFB laser 1 is regulated, there has arisen a problem of a shift of the wavelength of the carrier signal which is important in the DWDM communication system.
Moreover, since the characteristics of amplification rates of the plurality of optical amplifiers disposed on the transmission path in the DWDM communication system are varied according to the wavelength, there has been a possibility of deterioration of communication accuracy of the DWDM communication system.
An object of the present invention is to provide a DFB laser driving device, a DFB laser driving method and a storage medium for regulating an optical output level of a DFB laser without shifting a set wavelength.
Another object of the present invention is to strictly control an optical output level or wavelength of a DFB laser.